, in southern Italy In
recorded history, explosive eruptions at
subduction zone (
convergent-boundary) volcanoes have posed the greatest hazard to civilizations.
Subduction-zone stratovolcanoes, such as
Mount St. Helens,
Mount Etna and
Mount Pinatubo, typically erupt with explosive force because the
magma is too
viscous to allow easy escape of
volcanic gases. As a consequence, the tremendous internal pressures of the trapped volcanic gases remain and intermingle in the pasty
magma. Following the breaching of the
vent and the opening of the crater, the
magma degasses explosively. The magma and gases blast out with high speed and full force. It produced a large cloud of
volcanic ash that affected global temperatures, lowering them as much as 0.5 °C. The
eruption of Mount Vesuvius in 79 AD is the most famous example of a hazardous stratovolcano eruption. Pyroclastic surges completely smothered the nearby ancient cities of
Pompeii and
Herculaneum with thick deposits of ash and
pumice ranging from 6–7 meters deep. Pompeii had 10,000–20,000 inhabitants at the time of eruption.
Mount Vesuvius is recognized as one of the most dangerous of the world's volcanoes, due to its capacity for
powerful explosive eruptions coupled with the high population density of the surrounding
Metropolitan Naples area (totaling about 3.6 million inhabitants).
Ash 's ashfall deposits in a parking lot on
Clark Air Base (15 June 1991) In addition to potentially affecting the climate,
volcanic ash clouds from
explosive eruptions pose a serious hazard to
aviation. For example, during the 1982 eruption of
Galunggung in
Java,
British Airways Flight 9 flew into the ash cloud, causing it to sustain temporary engine failure and structural damage. Although no crashes have happened due to ash, more than 60, mostly
commercial aircraft, have been damaged. Some of these incidents resulted in emergency landings.
Lava in
Philippines extruding lava flows during its eruption on 29 December 2009
Lava flows from stratovolcanoes are generally not a significant threat to humans or animals because the highly
viscous lava moves slowly enough for everyone to evacuate. Most deaths attributed to lava are due to related causes such as
explosions and
asphyxiation from
toxic gas.
Lava flows can bury homes and farms in thick
volcanic rock which greatly reduces property value. Lava is typically between 700 and 1,200 °C (1,300–2,200 °F).
Volcanic bombs Volcanic bombs are masses of unconsolidated rock and lava that are ejected during an eruption. Volcanic bombs are classified as larger than 64mm (2.5 inches). Anything from 2 to 64mm is classified as
lapilli. When erupted, volcanic bombs are still molten and partially cool and solidify on their descent. They can form ribbon or oval shapes that can also flatten on impact with the ground. Volcanic bombs are associated with
Strombolian and
Vulcanian eruptions and
basaltic lava. Ejection velocities ranging from 200 to 400 m/s have been recorded causing volcanic bombs to be destructive. Depending on the abundance of volcanic debris the lahar can be fluid or thick like concrete.
Lahars have the strength and speed to flatten structures and cause great bodily harm, gaining speeds up to dozens of kilometers per hour.
SO2 classified as a respiratory, skin, and eye irritant if come into contact with. It is known for its pungent egg smell and role in ozone depletion and has the potential to cause acid rain downwind of an eruption.
H2S has an even stronger odor than
SO2 as well as being even more toxic. Exposure for less than an hour at concentrations of over 500 ppm causes death.
HF and similar species can coat
ash particles and once deposited can poison soil and water. Gases are also emitted during volcanic degassing, which is a passive release of
gas during periods of dormancy. ==Eruptions that affected global climate==